Rail mounted cranes and certain other types of machines that are mounted for travel through limited distances along define paths have trucks that comprise a tandem pair of flangeless wheels that roll along a single rail. To confine the flangeless wheels against movement off of the rail, each truck also comprises guide rollers rotatable on vertical axes, two at each side of the rail.
Such flangeless wheel trucks have significant advantages over trucks with flanged wheels. A wheel flange affords guidance to a rolling wheel by making sliding engagement with the rail, and it thus produces a substantial amount of friction that increases power requirements and causes wear on the wheel and the rail. A flangeless wheel truck, by contrast, produces only rolling friction and bearing friction, both of which are negligible in comparison to the sliding friction of a flanged wheel. Furthermore, the bearings for the flangeless wheels and for the guide rollers that cooperate with them need only support radial loads, whereas the bearings for a flanged wheel have to support substantial axial thrust loads as well as radial loads; hence, a flangeless wheel system can have less expensive bearings which nevertheless have a longer service life.
To some extent the advantages of a flangeless wheel system are lost if the flangeless wheels are out of alignment, that is, if their axes are not accurately parallel and transverse to the longitudinal centerline of the rail. It will be apparent that if the axis of a flangeless wheel is skewed to a substantial extent, the wheel tends to run off of the rail but is confined against doing so by the guide rollers, so that the wheel rolls along in a constant skid that creates power consuming friction, wears the wheel and the rail, and imposes an axial thrust load upon the wheel bearings as well as imposing an abnormally high radial load on the bearings for at least one of the rollers.
With these considerations in mind, it is obviously desirable to check the alignment of flangeless wheels, not only during the initial assembly of the system but also from time to time thereafter to detect and correct any misalignment that may have developed in service. Heretofore, however, there has been no simple and reliable means for accomplishing such checking.
Ordinarily, the wheels and rollers of a flangeless wheel truck are relatively inaccessible and are therefore somewhat difficult to see. Rotation of the guide rollers can afford important clues to the nature and degree of any misalignment, but the rollers are especially hard to observe when the truck is moving along the rail. Even if one or two rollers are visible, a satisfactory determination of the nature and degree of any wheel misalignment requires a knowledge of what all four rollers are doing during a movement of the truck, and one person cannot observe rollers on both sides of the rail at one and the same time.